Surface Plasmon Resonance (SPR) and microgravimetric sensing techniques, such as Quartz Crystal Microbalance (QCM), are known independently as methods suitable for in-situ, label-free sensing and analysis of binding reactions. Sensors using SPR or QCM have been used to analyse biological, biochemical and chemical samples.
Conventional QCM devices comprise a quartz crystal wafer having two planar metal electrodes disposed on the two surfaces of the wafer. The sample to be analysed is adsorbed onto the surface of one of the electrodes. The change in the quartz crystal can be excited to mechanical resonance by an alternating electric field due to the inverse piezoelectric effect. The oscillation frequency is dependent upon the mass and the viscoelastic property of material adsorbed onto the surface electrode. In general, the oscillation frequency decreases if mass accumulates and increases if mass reduces. In some instances (for example, where the adsorbed layer is rigid), the shift in the oscillation frequency can be related to the adsorbed mass using analytical equations. A mass loading in the order of about 1 ng/cm2 can be detected.
SPR is a known method for the detection of optical changes occurring at the surface of a thin metal film. SPR measures changes in the optical thickness (calculated by assuming a reasonable refractive index for the absorbed layer, may not be equal to the geometrical thickness) arising from molecular adsorption on the metal surface. In SPR, an evanescent wave (which is an exponential-decaying wave) presents at the sensor surface. An evanescent wave is generated when total internal reflection of incident light occurs at the interface of a substance with a high refractive index and a substance of low refractive index (e.g. a glass-air interface of a prism). SPR occurs under certain conditions when a thin film of metal (e.g. gold or silver) is placed on the bottom of a prism or one side of a planar substrate of the same material whose opposite side is attached to the bottom of the prism via a thin layer of an index-matched liquid. If the incident light is monochromatic, the free electrons of the metal will oscillate (i.e. surface Plasmon Polaritons are excited) and absorb light energy at a certain angle of incidence. The angle is called the SPR angle. The SPR signal is detected by measuring the intensity of the reflected light using a photodiode detector. With an appropriate metal thickness (˜47 nm for gold and ˜50 nm for silver) and a satisfied flatness (roughness less than a few nanometers), almost all light is coupled to excite Surface Plasmon Polaritons which “illuminate” at the surface and propagate along the surface at the resonance angle. As a consequence, the reflectivity drops to ˜0.
The position of the SPR angle depends on the optical property changes of the sensing surface due to the binding of molecules to the surface or the removal of the materials from the surface. The shift of SPR angle can be correlated to the amount of molecules adsorbed/desorbed at the surface by assuming a reasonable refractive index. The detection limitation of SPR is approximately 1 ng/cm2.
SPR and QCM techniques each have their own specific strengths, weaknesses and have assumptions that are inherent in data collection and analysis. Accordingly, each technique is sensitive to different properties of a thin film sample.
Analytical devices that combine both SPR and QCM techniques are known. These combinations, however, generate complexities which result in inaccurate or irreproducible experimental results. For example, when a piece of metal-electrode furnished quartz crystal wafer is placed in a QCM sample cell with a transparent sealing window (e.g., glass), interference of the reflected beams off the cell window and off the quartz crystal wafer surface adversely affects the intensity of the reflected light ultimately reaching the photodiode detector of the SPR analyzer. As a result, the SPR spectrum, i.e. a summary of the reflectivity data measured at each angle of incidence, would be based on the resultant superimposition of the unwanted reflected light off the cell window (glass) and the desired reflected light off the quartz crystal wafer surface, thereby roughening the spectrum and making accurate detection barely possible. Other stray light sources that would potentially interfere with the desired reflected light also pose a problem for these hybrid analytical systems.
Moreover, the setup of these hybrid analytic systems are fairly complicated and requires the presence of other components such as a lock-in amplifier, a photodiode detector, a detector motor, a frequency modulator (a light chopper) and a light polarizer(s) to be present. The need for the presence of a large amount of auxiliary components also portends a higher likelihood of achieving inaccurate results due to the increased number of additional parameters that have been introduced. Furthermore, the elaborate setup which follows when combining QCM and SPR makes it economically undesirable. This explains why the implementation of such hybrid systems is not common.
There is a need to provide a surface plasmon resonance sensing method that overcomes, or at least ameliorates, one or more of the disadvantages described above.
There is also a need to provide a method and system for combining surface plasmon resonance and gravimetric sensing, such as QCM, which avoids, or at least ameliorates, one or more of the disadvantages described above.